I am assuming I will need to buy the Thunderbolt 3 cable when I purchase my new iMac. To migrate from my mid 2010 iMac to the new iMac, what else will be required? There is no Thunderbolt portal on my 2010 iMac.

the thunderbolt 3 standard is only supposed to support its full capabilities in passive cables that are up to 0.5m long (40Gbps data transfer speeds, 4 lanes of PCIe Gen 3, 8 lanes of DisplayPort 1.2, full USB 3.1 Gen2, and 100W power delivery).
since this cable is 0.8m long, is it an active cable?

Short answer: It seems this is a passive cable. But I'm not 100% certain.

Long answer: After 45 minutes on a call with Apple support, the rep could not get a definitive answer, but we both concluded that this must be a passive cable for these two reasons:
1) The connector on the 0.8m Apple cable is fairly compact. It is much shorter than the connector on my active 2m and 0.5m TB3 cables from OWC. The longer connector hardware on the OWC cable would assumedly accommodate the active circuitry. OWC's connector hardware (between the USB C connector and the cable itself) is about twice as long as Apple's. Well, you may say Apple may be capable of making a much more compact active transceiver than OWC, so moving along...
2) Per Apple's specs, the Apple 0.8m TB3 cable is capable supporting USB 3.1 Gen 2. According to an apple insider article (link not provided as it prevents me to post this answer with an external link), indicates that only a passive TB3 cable can support USB 3.1 Gen 2. Once the active circuity is involved, the tradeoff is you lose the full 3.1 g2 USB speed.

Now what gets confusing is that a passive cable longer than 0.5m may not support the full 40Gb/s TB3 speed. But Apple does claim this cable to support 40Gb/s, so perhaps Apple discovered they achieve 40Gb/s at 0.8m, but to jump to 1m or 2m would result in slower speeds. For most users the distinction may not matter, but to the power user it does. I have an enclosure that RAIDs four NVMe M2 SSD chips which are extremely fast, so I want the full TB3 40Gb/s spec. As such, I either need to connect via a very short 0.5m passive or an active TB cable of up to 2m. If you want one cable that can give you both USB 3.1 get two speed but is also rated for the full 40Gb/s TB3 spec, then you ideally use a short passive TB3 cable, but 0.5m is very constraining.

No, full speed Thunderbolt 3 cables (40 Gbit/sec) that are 1m or longer can't support both full 40 Gbit/s Thunderbolt 3 speeds and USB 3.0/3.1 speeds. They and are limited to USB 2.0 when connected to USB devices. That is simply due to the physical limitations of the spec/cable materials.

For example, this is why Belkin's 2m 40 Gbit/sec Thunderbolt 3 cable is limited to USB 2.0 speeds when connected to USB devices.

This is likely why Apple chose the rather peculiar 0.8m length. It allows this cable to have the best of all three worlds, Thunderbolt 3 speeds up to 40 Gbit/sec, USB 3.1 speeds up to 10 Gbit/sec, and full 100W of power.

I'm actually pretty impressed, as far as I'm aware this is the first cable of its kind to reach 0.8 meters and it does it without being super bulky at the end.